1. Technical Field
The invention relates generally to the field of digital printing. More specifically, the invention relates to determining calibration values and color profile data as they pertain to digital printers.
2. Description of the Related Art
Color management is the art and science of managing devices and their output such that users experience consistency in color output. For example, a user may want to print an image on glossy paper and then print the same image on a newspaper. It isn't difficult to imagine that the resulting colors of the image on each respective type of paper would appear very different to the eye. A yellow sun and a blue sky rendered on the glossy paper one would imagine would be bright and cheerful. The same yellow sun and same blue sky rendered on a piece of newspaper one would imagine being rather dull and lifeless.
In another example, a user may print the image mentioned above on a particular printer and on glossy paper a day apart. One skilled in the art can imagine that the rendered colors yellow and blue on the first day may look different from the rendered colors on the second day As an example, the printer may print less cyan on Day 2. For a print of an image that includes a sky, the color of the sky may be different and lighter than when printing the same image on Day 1. Thus, given the same materials but over a lapse of time, the user may again experience inconsistency in rendered colors.
Achieving consistent digital printer output may be of great value to particular industries that depend on digital graphics at some stage of their business. For example, advertising agencies can suffer great losses if they do not deliver products with the colors that their clients expected. Graphic artists may end up throwing away expensive brochures if the colors on the first printed brochure are visibly different from the “same” colors on the last printed brochure.
Thus, color management has become its own industry with researchers and practitioners working at improving techniques within the industry for the benefit of the providers and the consumers.
Two particular aspects of color management in regard to digital printers are printer calibration and color profiles stored on the printers. Calibration is the process by which colors that are produced by a particular printer are adjusted according to particular references of colors for that particular printer. For example, it has been found that the printed colors from a particular printer drift over time. Thus, calibration is a process that compensates for printer drift over time in order to reach a known color reference. For example, after a particular period of time, one skilled in the art knows to calibrate the printer so that the expected color output is again achieved. As another example, consumers of printer cartridges for home printers know to calibrate the printer after they have just installed a new ink cartridge.
Color profiles are data that take into account the particular printer, the color settings, and the type of media, e.g. paper, used. More particularly, color profiles are International Color Consortium (ICC) color profiles. An ICC profile is a set of data that characterizes a color output device, according to standards published by the ICC. The color output varies with different media types and halftones.
In the example above about printing the image with the yellow sun and blue sky on glossy paper and on newspaper, using color profiles can ensure that the output of the colors look the same to the human eye. One profile sets the color output or signal to the printer for glossy paper and another profile sets the color output or signal to the printer for the newspaper.
More particularly, color profiles are used to do color conversion. For example to convert an image from RGB to CMYK so that the image can be printed on a particular printer, two ICC profiles are needed: the RGB profile and the printer CMYK profile. To perform the conversion, each RGB triplet is first converted to the profile connection space (PCS) using the RGB profile. Then the PCS is converted to the four values of CMYK required. The conversion from PCS to CMYK varies depending on the media type used in the printer and the halftone used to make the print.